Power supply

ABSTRACT

A power supply is described for use with an electron gun employed in an electron beam furnace system. The power supply includes a transformer and a rectifier connecting the secondary of the transformer to the electron beam gun for supplying current thereto. The power supply also includes a semiconductor-switching circuit connected to the primary of the transformer for supplying current to the primary. The switching circuit is controlled by a pulsing circuit, and a trigger circuit is controlled by a pulsing circuit, and a trigger circuit is connected to the pulsing circuit and to means for sensing a rise in current to the electron gun in the presence of an arc. Upon the sensing of such rise in current, the trigger circuit disables the pulsing circuit to thereby render the switching circuit inoperative and cut off current to the electron beam gun.

United States Patent [72] Inventor Emmett R. Anderson Berkeley, Calif.[21] Appl. No. 57,514 [22] Filed July 23, 1970 [45] Patented Sept. 28,1971 [73] Assignee Air Reduction Company, Incorporated New York, NY.Continuation-impart of application Ser. No. 675,902, Oct. 17, 1967, nowPatent No. 3,544,913. m [54] POWER SUPPLY 5 Claims, 2 Drawing Figs.

[52] 0.8. CI. 13/31 [51] Int. Cl 1105b 7/00 [50] FieldofSearch l3/l2,31;307/252.73, 252.53; 315/106; 219/121 EB [56] References Cited UNITEDSTATES PATENTS 3,165,571 1/1965 Grimes,Jr. 13/12X 3,400,207 9/1968Anderson 13/31 PULQC Cl RCUIT CIECU IT Primary Examiner-Bernard A.Gilheany Assistant Examiner-R. N Envall, Jr AttorneyFitch, Even, Tabin &Luedeka ABSTRACT: A power supply is described for use with an elec' trongun employed in an electron beam furnace system. The power supplyincludes a transformer and a rectifier connecting the secondary of thetransformer to the electron beam gun for supplying current thereto. Thepower supply also includes a semiconductor-switching circuit connectedto the primary of the transformer for supplying current to the primary.The switching circuit is controlled by a pulsing circuit, and a triggercircuit is controlled by a pulsing circuit, and a trigger circuit isconnected to the pulsing circuit and to means for sensing a rise incurrent to the electron gun in the presence of an arc. Upon the sensingof such rise in current, the trigger circuit disables the pulsingcircuit to thereby render the switching circuit inoperative and cut offcurrent to the electron beam gun.

POWER SUPPLY This application is a continuation-in-part of copendingapplication Ser. No. 675,902 filed Oct. I7, 1967 now US. Pat. No.3,544,9l3. This invention relates to electrical power supplies and, moreparticularly, to a power supply for an electron gun employed in anelectron beam furnace system.

The employment of electron beam furnace systems in various materialtreating processes such as melting, vapor plating, etc., has becomeincreasingly prevalent. In addition, an electron beam furnace system mayin some cases be employed as a pump, such as in the case of a titaniumsublimation pump in which titanium is sublimited and condensed within achamber to getter molecules of gas within the chamber and thereby reducethe pressure within the chamber.

A typical electron beam furnace system including those used for pumpingas above described includes an. electron gun, which is appropriatelyenergized to furnish a high intensity beam of electrons. The electrongun is generally disposed in an evacuated chamber together with thematerial to be heated, and means are provided for directing the electronbeam at the material. The electron gun usually, includes a source ofelectrons, such as a heated cathode or element, and a groundedaccelerating anode, the cathode being maintained at a high negativepotential with respect tothe anode so as to establish a highelectrostatic field for accelerating the electrons. A suitabletransverse magnetic field may also be provided for directing theelectrons onto the target material. As the beam of electrons impinges onthe target material, the material is heated, the amount of heatdeveloped being related to the electron beam current and the electronvelocity effected by the accelerating electrostatic field through whichthe electrons are directed.

During bombardment of the target material by the electron beam, variousvaporous materials are emitted and, in addition, various occluded gasesmay be released. The presence of such gaseous materials often effects adecrease in the resistance "between various parts of the electron beamgun and leads and surrounding elements. This may result in arcingbetween such parts and leads and elements, causing a substantialincrease in the electron gun current and possibly resulting in harm tothe electron gun structure and surrounding elements. To minimize theharmful effects of arcing, various voltage and currentregulatingelectron gun power supplies have been developed.

Some previously known electron gun power supplies for electron beamfurnace systems have limited the detrimental effects of arcing bylimiting or cutting back the current to the electron gun. By limitingthe current rise in the presence of an arc to a predetermined maximumvalve, the arc will often quickly terminate and normal operation may beresumed. For systems operating at relatively high-power levels, (such asto kilowatt systems operating with three or more amperes of beamcurrent) electron gun current may be restored without coincidentrestoration of the are merely by cutting back the electron gun currentsufficiently. in copending application Ser. No. 868,284, assigned to thepresent assignee, a power supply is disclosed in which electron guncurrent is cut back very quickly upon sensing the incipiency of an are.This starves the arc in its incipiency and thus enables restoration ofelectron gun current very quickly without coincident restoration of thearc.

Presently available power supplies have generally utilized vacuum tubedevices. Although satisfactory for many applications, some circumstancesmay make it desirable that the heat generated and the power supply sizeand weight be minimized. This naturally suggests the use of solid statedevices.

Because of the relatively high voltages and currents utilized in anelectron beam gun, the present state of the are does not permit meresubstitution of solid state devices for the vacuum tube devicespreviously utilized in electron gun power supplies. This is due to thepresent high cost or unavailability of satisfactory solid state devicesfor accomplishing functions performed by vacuum tubes at high voltagesand currents. Accordingly, design of a power supply incorporating allsolid state components involves solving the high voltage andcurrent-problems.

As previously mentioned, it is desirable to provide for rapid cut backof the electron beam current in the presence of arcing to thereby starvearcs in their incipiency and enable rapid restoration of beam currentwithout concurrent restoration of arcs. Some highly successful systemshave been designed to accomplish this in the case of relatively largefurnaces. In the case of a small furnace system, however, such as istypical in the case of a titanium sublimation pump, it is desirable todesign a simple, low cost and reliable power supply.

It is therefore an object of this invention to provide a power supplyfor an electron beam gun employed in an electron beam furnace system,such power supply being compact and light in weight.

Another object of the invention is to provide a power supply for anelectron beam gun employed in an electron beam fur.- nace system, suchpower supply utilizing solid state devices.

A further object of the invention is to provide a power supply of thetype described which is low in cost and reliable in operation.

It is another object of the invention to provide a power supply of thetype described and which operates to cut back electron beam current to alevel which starves incipient arcs, thereby permitting rapid restorationof beam current without concurrent restoration of arcs.

Other objects of the invention will become apparent to those skilled inthe art from the following description, taken in connection with theaccompanying drawings, wherein:

FIG. 1 is a schematic diagram illustrating an electron beam furnacesystem and illustrating a power supply, for the electron beam gun of thesystem, constructed in accordance with the invention; and

FIG. 2 is a schematic diagram of one type of pulsing and trigger circuitarrangement which may be utilized in the power supply of FIG. 1. Verygenerally, the power supply of the invention is employed with anelectron beam furnace system ll and includes a transformer 12 andrectifiers l3 connecting the secondary of the transformer to theelectron beam gun l4 of the furnace system. A switching circuit 15 isconnected to the primary of the transformer 12 and is adapted forconnection to a source of alternating current. A pulsing circuit 16 iscon nected to the switching circuit 15 for providing firing pulses tothe switching circuit for operating same. A trigger circuit 17 isconnected to the pulsing circuit 16 and is also connected to a sensingdevicel8. The sensing device senses a rise in current to the gun 1.4 dueto an arc. Upon the occurrence of such a current rise, the triggercircuit 17 operates to disable the pulsing circuit 16, thereby renderingthe switching circuit 15 nonconductive and cutting ofi current to theelectron gun 14.

Referring now more specifically to FIG. I, the invention is shownutilized in connection with an electron beam furnace system 11constituting a titanium sublimation pump. As is the case in otherelectron beam furnace systems, the titanium sublimation pumpincorporates an electron beam gun and a system for'directing the beam toheat ,a target, in this case a feed rod 21 of titanium. The titanium atthe end of the rod which is heated sublimates and condenses on suitablecollection means, not shown, to getter any molecules in the chamber,resulting in a pumping action. The operation of a titanium pump is wellknown to those skilled in the art and will not be discussed in furtherdetail herein.

The furnace system or titanium pump 11 includes the electron beam gun 14which is disposed within a vacuum-tight enclosure 23. The electron gun14 may be of any conventional type. In the illustrated embodiment, theelectron gun 14 includes a cathode 25 and a grounded accelerating anode27. The electrons emitted by the cathode 25 are accelerated and formedinto a beam 29 by the accelerating potential established betweenthecathode 25 and the grounded anode 27. The beam is deflected by atransverse magnetic field, produced by an appropriately positionedeiectromagnet 31, onto the upper .end of the titanium rod 21. Thecathode 25 may be heateddirectly or indirectly by suitable means, notillustrated. A shaping or backing electrode 33 is provided havingatrough 35 in which the cathode 25 is disposed. Because the shapingelectrode 33, like the cathode 25, is at a highly negative potential.the electrons are urged out of the open side of the trough 35 to bedeflected towards the target.

During operation of the foregoing described power supply, the voltagewhich is developed across the electron gun 14 may, if desired, begenerally held constant by a suitable voltage sensing and regulatingnetwork, not illustrated. Such voltage-regulating circuitry may includesuitable provision for preventing the occurrence of high current surgesresulting from switching transients generated during cut back of guncurrent, described below.

Power for operating the electron beam gun is obtained from a suitablethree-phase supply, not shown, the output terminals of which areindicated at 37, 39 and 41. The terminals 37, 39 and 41 are connectedthrough chokes 43, 54 and 47, respectively, to the anodes ofsemiconductor diodes 49, 51 and 53, respectively. The cathodes of thediodes 49, 51 and 53 are connected to terminals at the junction pointsof the delta connected primary windings 55, 57 and 59 of the transformer12. Semiconductor controlled rectifiers 61, 63 and 65 are connectedacross the diodes 49, 51 and 53, respectively, in reverse polarity. Withthe semiconductor-controlled rectifiers switching periodically intoconductive condition, the controlled rectifiers together with the diodes49, 51 and 53 constitute a switching circuit which transmits thethree-phase alternating current from the source to the primary windingsof the transformer 12. The chokes 43, 45 and 47 aid in the suppressionof switching transients.

The secondary of the transformer 12 consists of the secondary windings67, 69 and 71, connected in delta and inductively coupled to the primaryof the transformer through a suitable core 73. The secondary of thetransformer 12 is connected to the electron gun 14 through thethree-phase rectifier circuit 13. The three-phase rectifier circuitconsists of six diodes 75, 77, 79, 81, 83 and 85 connected to provide adirect current output to the cathode 25 of the electron gun. Thejunctions or terminals of the delta connected secondary winding of thetransformer 12 are connected to the junctures between the respectivepairs of diodes 75, 81; 77, 83; and 79, 85.

During normal operation of a high vacuum electron beam furnace system,arcing between various elements of the electron beam gun and leads andvarious elements of the furnace may periodically occur. Although theprecise conditions which produce arcing are not entirely understood, itis believed that local hot spots producing an increase in the level ofthermionic emission, and the presence of significant quantities ofpositive ions in a particular region, may contribute to the occurrenceof an arc.

An arc may be described generally as having two stages; an incipientstage which is manifested by a rapid rise in current to the electrongun, and a steady state stage in which the current stabilizes at a pointwhere the arc passes the maximum power. By merely limiting current to alevel below the higher current steady state stage, damage to gun andfurnace parts may often be prevented. In some cases, however, the arcmay continue at the lower current level and may rise to the highercurrent level steady state stage when current limiting is removed.

Accordingly, rather than current limiting, it is often necessary tosubstantially reduce the power supplied to the electron beam gun andmaintain it at the reduced level for a period of about four-tenths of asecond or more before power can be restored without coincidentrestoration of the arc. It is believed that this delay allows the largenumber of ions in the arcing region to dissipate throughout the vacuumfurnace and allows the regions which have been heated to a hightemperature and which may have a high level of thermionic emission, tocool down. A delay of four-tenths of a second or more is sig nificantand may contribute to a relatively high level of inefficiency in furnaceoperation and fluctuation in energy delivered to the material beingheated. The latter phenomenon can have a particularly deleterious effectin the case of vapor deposition operations, since it may produce anintolerable variation in the vapor deposition rate.

If, as taught in the aforementioned application Ser. No. 868,284, thearc is starved in itsincipient stage, by cutting back the electron guncurrent sufficiently before current can rise to the higher steady statelevel, full operating current may be restored very quickly withoutcoincident restoration of the arc. Although not entirely understood, itis believed that fast restoration is possible because extensiveionization of vapor particles in the region of the arc is avoided, orbecause extensive local superheating of electron emissive surfaces doesnot occur, or both. 1

In order to gain the benefit of fast tum-on, as has been previouslymentioned, electron gun current is cut back while the arc is in itsincipiency. Just how far ahead of the steady state condition, intime,.the cut back of current should occur depends upon the particularcircuit characteristics and component values, the degree of vacuum inthe electron beam furnace, the amount and kinds of vapors present aroundthe electron gun, and the particular geometry of the electron gun itselfand the surrounding furnace structure. With furnaces of relatively lowpower levels, if the electron gun current is cut back less than about 15milliseconds after the beginning of an arc, it is often possible torestore electron gun current within 200 milliseconds without coincidentrestoration of the arc.

Experience indicates that, for most furnace system configurations, theelectron gun current should be cut back to a minimum current level inorder to starve the arc. The level required for satisfactory operationis typically less than 2 amperes, and for high reliability it is oftenpreferable that it be cut back to less than l ampere.

A further advantage accrues from rapid cutoff of electron gun current atthe incipiency of an arc. This advantage stems from the fact that thepresence of an arc is usually accompanied by a high-level ofradiofrequency (RF) transients. The power supply circuitry may besensitive to such transients and complications may develop during theirpresence. RF traps may be included in the circuitry at suitablelocations to cut down the effect of the RF transients, but thisnaturally leads to an increase in the cost of the circuit. Because ofthe reduction in RF transients, flowing from the fact that the arcs arestarved in their incipiency, circuit design is simplified in thisrespect. In order to sense the incipiency of an arc, the resistor 18 isutilized. The voltage appearing across the resistor 18 is related to andindicative of the electron beam current, and the voltage is applied tothe SCR trigger circuit 17. Suitable means, not lllustrated, areincorporated in the trigger circuit in order to temporarily interruptthe operation of the pulse circuit 16 when the voltage across theresistor exceeds a preselected level corresponding to the incipientstage of an arc. Such means are readily designed by those skilled in theart and may, for example, be similar to the circuits shown in chaptereight of Semiconductor Controlled Rectifiers, by Gentry, Gutzwiller,Holonyak and von Zastrow; Prentiss Hall, 1964. The SCR pulsing circuit16 operates similarly to circuits taught in chapter five of theaforementioned publication to apply firing pulses to the gates of theSCRs 61, 63 and 65.

When operation of the circuit 16 is temporarily interrupted, thesemiconductor controlled rectifiers 61, 63 and 65 become nonconductive,cutting off alternating current to the primary of the transformer 12.This cuts 011' direct current supplied through the rectifiers 13 to theelectron gun 14, thereby cutting off beam current and starving any arcs.Using a 5 KC frequency for the output of the switching SCRs, as will beexplained below more fully, it is typically possible to cut back currentto the electron gun within about 8 milliseconds from the time guncurrent begins to rise. Gun current may then be restored in about I40milliseconds.

The SCR pulsing circuit 16 may be temporarily disabled, upon the sensingof a rise in current due to an arc, in any convenient way depending uponthe type of pulsing means used. For example, the trigger circuit mayoperate to remove bias for diode or transistor operation when thevoltage across the resistor 18 exceeds a preselected level. Anotherexample is the use of a diode or transistor shunt for the output of thepulsing circuit. Operating current for the SCR pulsing circuit isderived from a single-phase AC source, the terminals of which areindicated at 87 and 89.

Referring now to FIG. 2, one suitable arrangement of the pulsing circuit16 and trigger circuit 17 which may be utilized in the power supply ofFIG. 1, is illustrated. Alternating current from the AC source terminals87 and 89 is applied to a rectifier, DC amplifier and multivibratorcircuit 91 of suitable design. The circuit 91 is designed in accordancewith known practice to produce an output AC current superimposed on a DCbias current. Such an output may be achieved by using a regulated DCamplifier for amplifying the output of a bridge rectifier, and amultivibrator connected to shunt a part of the output of the DCamplifier to produce pulses at the frequency of the multivibrator.Output pulses of the circuit 91, which are superimposed on the regulatedDC and which are at the frequency desired for properly timing the SCRs61, 63 and 65 in the circuit of P10. 1 (e.g. 5 kc.), are passed to theemitter of an NPN transistor 93. Preferably, such frequency is about 4kc. to l0 kc. During normal operation, the transistor 93 is conductiveand the output pulses of the multivibrator 91 are therefore developedacross a diode 95 connected from the collector of the transistor 93 to areference line 97, which is at some suitable potential above ground.These pulses are then passed through three pulse transformers 99, 101and 103 to the gates ofthe SCR's 61, 63 and 65 in FIG. 1.

With the circuitry illustrated in FIG. 2, there is no intentionalcoordination of the switching frequency of SCRs 61, 63 and 65 with thefrequency of the AC source, nor is there any differential phase relationbetween the switching timing of the SCRs. Nevertheless, the SCRswitching frequency is high enough that a crude high-frequency AC signalis applied to the transformer for step-up and subsequent rectification.Alternatively, more sophisticated circuitry may be used includingprovision for a phase shift to trigger the SCRs 61, 63 and 65 atdifferent times, e.g. 120 out of phase. This may result in less ripplein the output of the rectifiers 13.

Biasing for the base of the transistor 93 is provided by a pair ofseries connected resistors 105 and 107 connected in series with thecollector emitter circuit of an NPN transistor 109 across the output ofthe circuit 91. A resistor 111 and a choke 113 are connected across thebase emitter circuit of the transistor 109. The junction between thechoke 113 and the resistor 111 is connected in the emitter base twocircuit of a unijunction transistor 115. The emitter of the unijunctiontransistor 115 is connected through a variable resistor 117 to theoutput of the circuit 91. A capacitor 119 connects the emitter of theunijunction transistor 115 to the other side of the circuit 91, and aresistor 121 connects the base one of the unijunction transistor 115 tothe output of the circuit 91.

During normal operation, the transistor 109 is maintained in theconductive state to maintain the transistor 93 in the conductive statethrough the biasing provided by the emitter to base two conduction ofthe unijunction transistor 115. As will be explained, when a rise incurrent to the electron beam gun due to an arc is sensed across theresistor 17, the unijunction transistor 115 is rendered nonconductive.The potential on the base of the transistor 109 then drops to a cutoffpotential and the potential on the base of the transistor 93 rises to aeutoff potential. Pulses are then prevented from reaching the gates ofthe SCRs 61, 63 and 65, thereby cutting off current to the electron beamgun.

in order to provide for disabling of the pulsing circuit for the SCRgates, a trigger circuit is provided. The trigger circuit includes achoke 123, a variable resistor 125 and a Zener diode 127 connected tothe resistor 17. A resiston-129 is connected across the Zener diode 127.The tap of the variable resistor 125 is connected through a resistor 131to the gate of a semiconductor controlled rectifier 133. The SCR 133 isseries connected with a resistor 135 across the output of the circuit91. A diode 137 connects the junction between the capacitor 119 and thevariable resistor 117 to the junction between the resistor 135 and theSCR 133. A resistor 139, a capacitor 141, and an inductor 143 areconnected in series across the SCR 133.

The voltage level setting on the variable resistor determines the levelof gun current at which the SCR 133 is rendered conductive. The triplevel is selected to correspond to a rise in gun current just sufficientto indicate that an arc is occurring. The trip level may be, forexample, 5 or 10 percent higher than the normal operating current. Whenthe voltage across the resistor 17 rises above the trip level, and theSCR 133 becomes conductive, current is shunted from the emitter of theunijunction transistor 115. The Zener diode protects the gate of the SCR133 by breaking down if the voltage thereon becomes excessive. Oncecurrent is removed from the emitter of the transistor 115, thattransistor is turned off and thereby turns the other two transistors 109and 93 off to disable the power supply. As this occurs, a currentdischarge from the capacitor 141 flows through the SCR 133 and theinductor 43. When this is complete, the inductor produces a reversespike in the discharge circuit which appears across the SCR and which issufficient to turn the SCR off. Current from the circuit 91 then flowsthrough the variable resistor 117, the diode 137 and the resistor 139 tocharge the capacitor 141. Once the capacitor charges to a levelsufficient to turn on the unijunction transistor 115, normal operationof the circuit resumes and current to the electron gun 14 is restored.The timing of this process is adjusted by suitably adjusting thevariable resistor 117.

Although the circuits described herein refer to semiconductor controlledrectifiers, it will be apparent to those skilled in the art thatsuitable transistor switches orother solid-state semiconductingswitching devices may be utilized in a manner within the teaching of thepresent invention.

It may therefore be seen that the invention provides a power supply, foran electron beam gun employed in an electron beam furnace system, whichis compact and light in weight. The power supply utilizes solid-statecomponents and is low in cost and reliable of operation. Provision ismade for cutting back electron beam current in the presence of arcing.

Various modifications of the invention other than those shown anddescribed herein will become apparent to those skilled in the art fromthe foregoing description and accompanying drawings. Such modificationsare intended to fall within the scope of the appendant claims.

What is claimed is:

1. A power supply for an electron gun employed in an electron beamfurnace system wherein the gun current is susceptible of rising upon theoccurrence of an arc, comprising, a transformer, rectifier means forcoupling the secondary of said transformer and adapted for connection toa source of alternating current, a pulsing circuit connected to saidswitching circuit for providing pulses thereto for operating saidswitching means, means for sensing a rise in current to the electron gundue to an arc, and a trigger circuit connecting said sensing means tosaid pulsing circuit, said trigger circuit being responsive to a sensingof a rise in current to the electron gun due to an arc by said sensingmeans to disable said pulsing circuit and thereby render said switchingmeans inoperative, said trigger circuit including a time delay circuitfor enabling said pulsing circuit after a predetermined time delay toreinitiate operation of said pulsing circuit and resume operation ofsaid switching means.

2. A power supply according claim 1 wherein said semiconductor-switchingmeans include a plurality of controlled rectifiers, and wherein saidpulsing circuit is connected to the gates of said controlled rectifiers.

3. A power supply according to claim 1 wherein said trigger circuitincludes means for shunting operating current from said pulsing circuitwhen said sensing means senses a rise in current due to an arc.

4. A power supply according to claim 1 wherein said trigger circuitincludes a semiconductor controlled rectifier connected toshunt-operating current from said pulsing circuit when saidsemiconductor controlled rectifier is conductive.

5. A power supply according to claim 1 wherein the output frequency ofsaid pulsing circuit is of the order of about 5 kc.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent NO.316091200 Dated September 28, 197].

Inventor(s) Emmett R. Anderson It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Abstract of Patent, lines 8-9, delete extra sentence "and a triggercircuit is controlled by a pulsing circuit,"

Column 1, line 48 "valve" should be -value-.

Column 1, line 67, change "are" to -art-.

Column 3, line 15, change 54" to 45.

Column 6, line 47, after "transformer" insert to the electron beam gun,semiconductor switching means connected to the primary of saidtransformer-.

Signed and sealed this 9th day of May 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents DRM F G-1050 (10-69) USCOMM-DC GOIITS-PUQ w u 5 GOVERNMENTPRINTING OFFICE 1969 O366-334

1. A power supply for an electron gun employed in an electron beamfurnace system wherein the gun current is susceptible of rising upon theoccurrence of an arc, comprising, a transformer, rectifier means forcoupling the secondary of said transformer to the electron beam gun,semiconductor-switching means connected to the primary of saidtransformer and adapted for connection to a source of alternatingcurrent, a pulsing circuit connected to said switching circuit forproviding pulses thereto for operating said switching means, means forsensing a rise in current to the electron gun due to an arc, and atrigger circuit connecting said sensing means to said pulsing circuit,said trigger circuit being responsive to a sensing of a rise in currentto the electron gun due to an arc by said sensing means to disable saidpulsing circuit and thereby render said switching means inoperative,said trigger circuit including a time delay circuit for enabling saidpulsing circuit after a predetermined time delay to reinitiate operationof said pulsing circuit and resume operation of said switching means. 2.A power supply according to claim 1 wherein said semiconductor-switchingmeans include a plurality of controlled rectifiers, and wherein saidpulsing circuit is connected to the gates of said controlled rectifiers.3. A power supply according to claim 1 wherein said trigger circuitincludes means for shunting operating current from said pulsing circuitwhen said sensing means senses a rise in current due to an arc.
 4. Apower supply according to claim 1 wherein said trigger circuit includesa semiconductor controlled rectifier connected to shunt-operatingcurrent from said pulsing circuit when said semiconductor controlledrectifier is conductive.
 5. A power supply according to claim 1 whereinthe output frequency of said pulsing circuit is of the order of about 5kc.